The present invention relates to high dielectric constant type ceramic compositions, and, in particular, to high dielectric constant type ceramic compositions which are capable of sintering at low temperatures, which have excellent electrical characteristics such as high insulation resistance and which are suitable for producing multilayer ceramic capacitors or the like. And this invention relates to multilayer ceramic capacitors produced by using such compositions as dielectrics.
Electrical characteristics which must be considered for dielectric materials include dielectric constant, temperature coefficient of dielectric constant (T.C.C.), dielectric loss, bias electric field dependence of dielectric constant, capacitance-resistance product and the like.
In particular, it is required that the capacitance-resistance product (CR product) be amply high. For example, Standards of Electronic Industries Association of Japan (EIAJ) stipulates, in the standards RC-3698B, entitled "Multilayer ceramic capacitors (chip-type) for electronic equipment", that the CR product be at least 500 M.OMEGA..multidot..mu.F at room temperature. Further it is required to maintain the high CR product even at higher temperatures so that capacitors can be used under even more severe conditions. (For example, Military Specification MIL-C-55681B stipulates a CR product at 125.degree. C.)
In the case of multilayer type elements, it is necessary to use internal electrode materials which can withstand even at the sintering temperatures of dielectric materials, because the electrode layers and the dielectric layers are co-fired. Accordingly, if the sintering temperatures of the ceramic dielectric materials are high, expensive precious metals such as platinum (Pt) or palladium (Pd) must be used as internal electrodes not to react mutually. Therefore, a requirement is that sintering be possible at lower temperatures of the order of 1100.degree. C. or below so that inexpensive metal such as silver (Ag) based alloy can be used.
A conventional high dielectric constant type ceramic composition is a solid solution containing barium titanates (BaTiO.sub.3) as the base materials and stannates, zirconates, titanates, etc. as additives. It is certainly possible to obtain a composition having a high dielectric constant, but such a composition has problems. If the dielectric constant becomes high, then T.C.C. becomes large. Further, the bias electric field dependence becomes large. Furthermore, since the sintering temperatures of the BaTiO.sub.3 -based materials are as high as 1,300.degree. to 1,400.degree. C., high-temperature resistant expensive precious metals such as platinum or palladium should be used for the internal electrode materials, thus resulting in cost augmentation.
In order to overcome the above mentioned problems of the BaTiO.sub.3 -based materials, extensive studies are being carried out on a variety of low-firing type compositions. For example, Japanese Patent Laid-Open Pub. No. 57204/1980 discloses a Pb(Fe.sub.1/2 Nb.sub.1/2)O.sub.3 -based composition; Japanese Patent Laid-Open Pub. No. 51758/1980 discloses a Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 -based composition; and Japanese Patent Laid-Open Pub. No. 21662/1977 discloses a Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3 -based composition.
The Pb(Fe.sub.1/2 Nb.sub.1/2)O.sub.3 -based composition is accompanied by the following problems. The sintering temperature dependence of the CR product is quite large. Particularly, the decrease of the CR product at a higher temperature such as at 85.degree. C. is large. The Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 -based composition requires a relatively high sintering temperature. Further, the Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3 -based composition is accompanied by the following problems. If the CR product is large, then the dielectric constant is small. If the dielectric constant is large, then the CR product is small. Furthermore, the T.C.C. of these materials is superior to that of the barium titanate, but it is insufficient.
Further, Japanese Patent Laid-Open Pub. No. 121959/1980 discloses a composition comprising a solid solution of Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 and lead titanate wherein if necessary a portion of Pb, less than 10 mole %, is substituted by barium, strontium or calcium. However, the T.C.C. of this composition cannot be said to be sufficient, the T.C.C. of the best composition being -59.8% at a temperature range of from -25.degree. to 85.degree. C. Further, Japanese Patent Laid-Open Pub. No. 121959/1980 mentioned above does not describe the CR product which is the most important property of a capacitor material, whereby the usefulness of this composition as a capacitor material is uncertain.
Still further, Japanese Patent Laid-Open Pub. No. 25607/1982 discloses a solid solution of Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 and Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3. However, this publication describes neither the CR product nor the T.C.C. Thus, the usefulness of this material as a capacitor material is also uncertain.
An object of the present invention is therefore to provide a high dielectric constant type ceramic composition wherein the following problems of the prior art are overcome:
(1) the sintering temperature is high;
(2) when the dielectric constant is large, the CR product is small; and
(3) the CR product (insulation resistance) at high temperatures is small; wherein the dielectric constant and insulation resistance are high; wherein such a composition can be sintered at low temperatures; and wherein it has excellent electrical characteristics.
Another object of the present invention is to provide a multilayer ceramic capacitor produced through the use of such a composition.